although only rarely in United States municipal-waste, hazardous-waste, and medical-waste incinerators. They are typically not as efficient as spray-dryer absorbers at removing emissions. The important design and operating criteria for spray-dryer absorbers and dry-alkali scrubbers include gas temperature in the reagent contacting zone, reagent-to-acid gas stoichiometry, reagent distribution in the gas, and reagent type.

NO_x emissions can be reduced by combustion-furnace designs, combustion-process modifications, or add-on controls. Combustion-furnace designs that reduce thermal NO_x include a variety of grate and furnace designs, bubbling and circulating fluidized-bed boilers, and boiler designs, especially those with automatic controls, that permit flue-gas recirculation. Combustion-process modifications that reduce NO_x formation include controlling the amount of oxygen available during the combustion process, and operating within a specific temperature range. For minimizing NO_x production in the combustion process, it is recommended that there be a lower-oxygen condition just above the grates (or in the primary chamber of a dual-chamber facility) coupled with a higher excess-oxygen condition at the location of overfire air injection (or in the secondary chamber of a dual-chamber facility). Municipal solid-waste incineration facilities tend to create the most NO_x when furnace temperatures are higher than is necessary (higher than 2,000°F) to destroy products of incomplete combustion (PICs). To minimize NO_x formation, and the formation of PICs, the furnace should be operated within fairly narrow ranges of temperature and excess oxygen (9-12%) with turbulent (well-mixed) conditions.

Some NO_x formation is inevitable from nitrogen present in the fuel and from atmospheric nitrogen, and it may be necessary to use flue-gas controls to achieve further reduction of these emissions. Add-on NO_x flue-gas control systems include selective noncatalytic reduction (SNCR), selective catalytic reduction (SCR), and wet flue-gas denitrification.

SNCR reduces NO_x by injecting ammonia or urea into the furnace via jets positioned at the location where temperatures are about 1600-1800°F. In the proper temperature range, the injected ammonia or urea combines with nitrogen oxide to form water vapor and elemental nitrogen.

SCR operates at a lower flue gas temperature than SNCR, and in addition uses a catalyst. Ammonia is injected into the flue gases when they are at about 600°F, and the mixture is passed through a catalyst bed. The catalyst bed may be shaped in a variety of forms (honeycomb plates, parallel ridged plates, rings, tubes, and pellets), while the catalyst can be one of a variety of base metals (such as copper, iron, chromium, nickel, molybdenum, cobalt, or vanadium). Each combination has advantages and disadvantages with respect to catalyst-to-NO_x contact, fouling of the catalyst, and pressure drop through the catalyst. The